FIGS. 1 and 2 illustrate an example of a prior art real-time X-ray fluoroscopic imaging system. Such systems may be used in the medical field to view interior portions of a patient's body without the need for invasive surgery. The fluoroscopic imaging system of FIG. 1 comprises an X-ray generator 101 and X-ray tube 102 which generate a radiation beam 103 which may be collimated and directed at an input surface of X-ray image intensifier 104. Patient 105 may be placed in the path of radiation beam 103, modulating radiation beam 103 before it strikes X-ray image intensifier 104 and thus generating a real-time image.
Image intensifier 104 converts modulated radiation beam 103 to a visible image which may be optically coupled to closed circuit television (CCTV) camera 106 and/or film camera 107 through optical distributor 108. X-ray Image intensifier 104 is an electro-optical device which converts X-ray radiation into a visible image. Intensification of the image may be accomplished by both a geometric reduction in image size and amplification via electron optics.
CCTV camera 106 converts the visible image into an electronic signal which may be transferred to a television monitor 201 and/or video recorder 202 for observation and/or recording, respectively. In addition, the video signal from CCTV camera 106 may be modified or enhanced by real time video image processor 203 which may be located at any point between CCTV camera 106 and television monitor 201 and/or video recorder 202. Film camera 107 may be used to record either multiple images (e.g., cine camera) or single image (e.g., still camera). Magnetic disk Storage 205 may be used to store data representing a frame or frames of video image for later retrieval and display.
During observation or recording of real-time images using CCTV camera 106, the radiation exposure level is relatively low, and due to the quantum nature and statistical uncertainty in energy levels of the X-ray photons, the signal-to-noise ratio of X-ray beam 103 may be relatively poor in comparison to a film image created by employing much greater quantities of radiation. This poor signal-to-noise ratio impairs the diagnostic quality of the resulting television image. The largest component of this "quantum noise" is temporal in nature, i.e., it varies more in time than in space.
Traditional methods for improving this temporal noise has been to use a television camera pick-up tube (e.g., vidicon tube) with a suitable amount of integration (time lag) which effectively reduces the amount of quantum noise to an acceptable level. Over the years of development of television camera systems used in fluoroscopy, this integrating characteristic of camera pick-up tubes has reached a high level of refinement resulting in a diagnostically acceptable video signal-to-noise ratio at relatively low radiation exposure levels.
Television cameras which use solid state sensors (e.g., charge coupled device (CCD) or the like) are now being used in fluoroscopy. Such solid state devices, however, have relatively no integration relative to a tube type pick-up sensor. As a consequence, some form of electronic image processing may be desirable to integrate video signals to improve the signal-to-noise ratio. Since the primary noise component is temporal, and the signal is video, the image processor must integrate in the time domain and at the video frame rate of the television system. Such image processing may be typically accomplished by employing a digital recursive filter.
Different types of fluoroscopic procedures and changing conditions during an individual procedure, however, require different amounts of temporal integration in order to obtain the highest degree of diagnostic image quality. Temporal integration improves the signal-to-noise ratio of an image, but may degrade temporal response, causing a "smearing" of moving objects. Consequently, more integration may be employed when there is little motion within an image. Prior art recursive filters may be provided with a motion detector and/or a manual selector to vary the amount of integration. Such techniques, however, do not accurately mimic the action of a pickup tube sensor.
A pick-up tube sensor's degree of integration varies primarily with the brightness of an image. This phenomenon may be observed, for example, in films of black and white television programs filmed using vidicon tubes. Bright images such as reflections or the like tend to integrate more and thus may be observed as a smearing effect or afterimage. In fluoroscopy, this integrating effect is one that physicians and technicians have become accustomed to in the over thirty years of use of the tube type sensors in X-ray fluoroscopy. The integrating effect of the tube type sensor is automatic, requiring no input from the observing physician or technicians.
FIGS. 3 and 4 illustrate the operation of recursive filtration (i.e., time based integration) versus spatial integration. Recursive filtration is a form of image processing in which every new image may be summed with a fractional combination of previous images to form an actual observable image. Recursive filtration is a temporal (i.e., time based), in other words, each pixel in the most recent image frame may be summed with a fractional combination of every geometrically corresponding pixel of all previously occurring image frames to form the presently observable image frame. Thus, each pixel in a recursive filter may be an integral of all preceding geometrically corresponding pixels. This type of filter may be distinct from a spatial filter in which each pixel in the most recent image frame may be integrated with a number of surrounding pixels in the same image frame to form a presently observable image frame.
A spatial filter integrates over space, while a recursive filter integrates over time. A variable recursive filter changes the fractional weighting of previous image frames and the present image frame in order to change the degree of integration present in the presently observable image frame. In order to achieve a longer time integral, greater weight may be given to previous image frame pixel values. Conversely, in order to achieve a shorter time integral, more weight may be given to the present image frame pixel values.